Control system and method

ABSTRACT

Embodiments of the present invention provide a system comprising: a first controller configured in each of a predetermined plurality of states to generate a torque request signal in order to cause a vehicle to operate in accordance with a target speed value, the system being configured, when the first controller is one of said plurality of states to generate a gear shift limit signal to cause a change in an engine speed at which a transmission controller is permitted to cause a gear downshift, the gear shift limit signal being generated in dependence on at least one traction indicator signal indicative of an amount of estimated traction between the vehicle and a driving surface.

INCORPORATION BY REFERENCE

The content of UK patent applications GB2492748, GB2492655 and GB2499252is incorporated herein by reference.

TECHNICAL FIELD

The present invention relates to vehicle speed control systems. Inparticular but not exclusively the invention relates to monitoring ofvehicle speed control systems to ensure correct operation.

BACKGROUND

In known vehicle speed control systems, typically referred to as cruisecontrol systems, the vehicle speed is maintained on-road once set by theuser without further intervention by the user so as to improve thedriving experience for the user by reducing workload.

With typical cruise control systems, the user selects a speed at whichthe vehicle is to be maintained, referred to as a set-speed, and thevehicle is maintained at a target speed that is set equal to theset-speed for as long as the user does not apply a brake or, in the caseof a vehicle having a manual transmission, depress a clutch pedal. Thecruise control system takes its speed signal from a driveshaft speedsensor or wheel speed sensors. When the brake or the clutch isdepressed, the cruise control system is disabled so that the user canoverride the cruise control system to change the vehicle speed withoutresistance from the system. If the user depresses the accelerator pedalby a sufficient amount the vehicle speed will increase, but once theuser removes his foot from the accelerator pedal the vehicle reverts tothe pre-set cruise speed (set-speed) by coasting.

Such systems are usually operable only above a certain speed, typicallyaround 15-20 kph, and are ideal in circumstances in which the vehicle istravelling in steady traffic conditions, and particularly on highways ormotorways. In congested traffic conditions, however, where vehicle speedtends to vary widely, cruise control systems are ineffective, andespecially where the systems are inoperable because of a minimum speedrequirement. A minimum speed requirement is often imposed on cruisecontrol systems so as to reduce the likelihood of low speed collision,for example when parking. Such systems are therefore ineffective incertain driving conditions (e.g. low speed) and are set to beautomatically disabled in circumstances in which a user may not considerit to be desirable to do so.

More sophisticated cruise control systems are integrated into the enginemanagement system and may include an adaptive functionality which takesinto account the distance to the vehicle in front using a radar-basedsystem. For example, the vehicle may be provided with a forward-lookingradar detection system so that the speed and distance of the vehicle infront is detected and a safe following speed and distance is maintainedautomatically without the need for user input. If the lead vehicle slowsdown, or another object is detected by the radar detection system, thesystem sends a signal to the engine or the braking system to slow thevehicle down accordingly, to maintain a safe following distance.

Known cruise control systems also cancel in the event that a wheel slipevent is detected requiring intervention by a traction control system(TC system or TCS) or stability control system (SCS). Accordingly, theyare not well suited to maintaining vehicle progress when driving in offroad conditions where such events may be relatively common.

It is an aim of embodiments of the present invention to addressdisadvantages associated with the prior art.

SUMMARY OF THE INVENTION

Embodiments of the invention may be understood with reference to theappended claims.

Aspects of the present invention provide a system, a vehicle and amethod.

In one aspect of the invention for which protection is sought there isprovided a system comprising:

-   -   a first controller configured in each of a predetermined        plurality of states to generate a torque request signal in order        to cause a vehicle to operate in accordance with a target speed        value,    -   the system being configured, when the first controller is one of        said plurality of states to generate a gear shift limit signal        to cause a change in an engine speed at which a transmission        controller is permitted to cause a gear downshift, the gear        shift limit signal being generated in dependence on at least one        traction indicator signal indicative of an amount of estimated        traction between the vehicle and a driving surface.

Optionally, the gear shift limit signal is further configured to suspenda gear downshift if the estimated traction between the vehicle and adriving surface is below a first threshold.

Optionally, suspending a gear downshift comprises reducing the enginespeed at which a transmission controller is permitted to cause a geardownshift to zero whilst the vehicle is in motion.

This feature has the advantage that a problem that locking of one ormore wheels driven by a powertrain due to a downshift when operating onsurfaces offering relatively low traction may be reduced.

It is to be understood that the amount of tractive force may be afunction at least in part of a surface coefficient of friction between adriving surface and a wheel, and a normal force on the wheel being aforce on the wheel normal to the driving surface.

Optionally, the gear shift limit signal is configured to reduce theengine speed at which a transmission controller causes a downshift.

Optionally, the gear shift limit signal is configured to substantiallyprevent a transmission controller from causing a downshift by reducingthe engine speed at which a transmission controller causes a downshift.

Optionally, the at least one traction indicator signal is a signalindicative of an amount of torque that may be applied to a wheel beforeslip of the wheel occurs.

Optionally, at least one said at least one traction indicator signal isindicative of a normal force between a wheel and a driving surface.

It is to be understood that, the lower the normal force on a wheel, thelower the tractive force that may be developed between a wheel and adriving surface. Similarly, the lower the surface coefficient offriction between a wheel and a driving surface, the lower the tractiveforce that may be developed.

Optionally, at least one said at least one traction indicator signal isindicative of a surface coefficient of friction between a wheel and adriving surface.

Optionally, the first controller is configured to perform a mu-checkoperation when in one of the plurality of states, the system beingconfigured to perform a mu-check operation by causing a transmissioncontroller to cause a downshift and monitoring a speed of one or morewheels in response to the downshift.

This feature has the advantage that the first controller may obtain anindication of the value of surface coefficient between a wheel and adriving surface.

Optionally, when performing a mu-check operation the first controller isconfigured to calculate an estimated value of surface coefficient offriction between a wheel and a driving surface in dependence on a changein a speed of one or more wheels in response to the downshift.

The system may take into account a change in speed of one or more wheelsdue to a reduction in speed of the ground when estimating a value ofsurface coefficient of friction. For example, in some embodiments if achange in wheel speed corresponds substantially to a change in vehiclespeed over ground when a mu-check is performed the first controller maydetermine that the surface coefficient of friction correspondssubstantially to unity. In contrast, if wheel speed reduces more rapidlythan vehicle speed over ground, for example if wheel speed fallssubstantially to zero whilst the vehicle is still moving over ground,the system may determine that the surface coefficient of friction isless than unity.

Optionally, the system may be configured to perform a mu-check operationwhen one or more predetermined conditions are met during operation inone of said plurality of states.

Optionally, the system may be configured to perform a mu-check atpredetermined intervals of time.

Optionally, the system may be configured to perform a mu-check followinga torque control intervention event in which a torque interventionsystem intervenes to cause change in an amount of torque applied to oneor more wheels.

It is to be understood that the torque invention system may be a systemthat is configured to intervene and cause a change in an amount oftorque applied to one or more wheels under certain predeterminedconditions, not being configured to cause a vehicle to operate inaccordance with a target speed value. For the present purposes a torqueintervention system performs functionality that is in addition to thatof the first controller. Examples of torque intervention systems includetraction control systems (TCS), stability control systems (SCS) andanti-lock braking systems (ABS).

Optionally, the system may be configured wherein the at least onetraction indicator signal is indicative of an amount of torque that maybe applied to a wheel before slip of the wheel slip exceeding apredetermined amount occurs.

Optionally, the system may be further configured, when causing a vehicleto operate in accordance with the target speed value, to generate abrake request signal to command a brake system to deliver an amount ofbrake torque.

Optionally, the system may be configured wherein said plurality ofstates includes a first one or more first states in which the system isconfigured to generate a powertrain request signal in order to cause apowertrain to deliver drive torque and cause a vehicle to operate inaccordance with a target speed value, and a second state wherein thesystem is configured to cause a vehicle to operate in accordance with atarget speed value by generating the brake request signal and not togenerate a powertrain request signal in order to cause a powertrain todeliver drive torque.

It is to be understood that the system may generate a nominal powertrainrequest signal even when in the second state, for example a request foran amount of torque corresponding to a released position of anaccelerator pedal. However the first controller does not actively causea powertrain to develop positive drive torque, that is the firstcontroller does not cause an increase in the amount of any positivedrive torque developed by a powertrain, or a reduction in an amount ofnegative torque developed by a powertrain. Other arrangements are alsouseful.

In one aspect of the invention for which protection is sought there isprovided a motor vehicle comprising a system according to anotheraspect.

In one aspect of the invention for which protection is sought there isprovided a vehicle comprising a chassis, a body attached to saidchassis, a plurality of wheels, a powertrain to drive said wheels, abraking system to brake said wheels, and a system according to anotheraspect.

In one aspect of the invention for which protection is sought there isprovided a method of operating a vehicle implemented by means of acontrol system comprising:

-   causing a powertrain to deliver drive torque and cause a vehicle to    operate in accordance with a target speed value;    -   generating a gear shift signal to cause a change in a speed at        which a transmission is permitted to downshift or substantially        prevent a transmission from downshifting,-   the method comprising generating the gear shift signal in dependence    on at least one traction indicator signal being a signal indicative    of an amount of tractive force that may be developed between a    driving surface and one or more driving wheels.

In one aspect of the invention for which protection is sought there isprovided a carrier medium carrying computer readable code forcontrolling a vehicle to carry out the method of another aspect.

In one aspect of the invention for which protection is sought there isprovided a computer program product executable on a processor so as toimplement the method of another aspect.

In one aspect of the invention for which protection is sought there isprovided a computer readable medium loaded with the computer programproduct of another aspect.

In one aspect of the invention for which protection is sought there isprovided a processor arranged to implement the method of another aspect.

It is to be understood that the controller or controllers describedherein may comprise a control unit or computational device having one ormore electronic processors. The system may comprise a single controlunit or electronic controller or alternatively different functions ofthe controller may be embodied in, or hosted in, different control unitsor controllers. As used herein the term “control unit” will beunderstood to include both a single control unit or controller and aplurality of control units or controllers collectively operating toprovide the stated control functionality. A set of instructions could beprovided which, when executed, cause said computational device toimplement the control techniques described herein. The set ofinstructions could be embedded in said one or more electronicprocessors. Alternatively, the set of instructions could be provided assoftware to be executed on said computational device. The controller maybe implemented in software run on one or more processors. One or moreother controllers may be implemented in software run on one or moreprocessors, optionally the same one or more processors as thecontroller. Other arrangements are also useful.

Within the scope of this application it is envisaged that the variousaspects, embodiments, examples and alternatives, and in particular theindividual features thereof, set out in the preceding paragraphs, in theclaims and/or in the following description and drawings, may be takenindependently or in any combination. For example features described inconnection with one embodiment are applicable to all embodiments, unlesssuch features are incompatible.

For the avoidance of doubt, it is to be understood that featuresdescribed with respect to one aspect of the invention may be includedwithin any other aspect of the invention, alone or in appropriatecombination with one or more other features.

BRIEF DESCRIPTION OF THE DRAWINGS

One or more embodiments of the invention will now be described, by wayof example only, with reference to the accompanying figures in which:

FIG. 1 is a schematic illustration of a vehicle according to anembodiment of the invention in plan view;

FIG. 2 shows the vehicle of FIG. 1 in side view;

FIG. 3 is a high level schematic diagram of an embodiment of the vehiclespeed control system of the present invention, including a cruisecontrol system and a low-speed progress control system;

FIG. 4 is a schematic diagram of further features of the vehicle speedcontrol system in FIG. 3;

FIG. 5 illustrates a steering wheel and brake and accelerator pedals ofa vehicle according to an embodiment of the present invention;

FIG. 6 is a schematic illustration of a known key fob for use with thevehicle of FIG. 1;

FIG. 7 is a schematic illustration of a portion of a controlleraccording to an embodiment of the present invention; and

FIG. 8 is a plot of gearshift points as a function of acc_pos and engineoutput shaft speed.

DETAILED DESCRIPTION

References herein to a block such as a function block are to beunderstood to include reference to software code for performing thefunction or action specified which may be an output that is providedresponsive to one or more inputs. The code may be in the form of asoftware routine or function called by a main computer program, or maybe code forming part of a flow of code not being a separate routine orfunction. Reference to function block is made for ease of explanation ofthe manner of operation of embodiments of the present invention.

FIG. 1 shows a vehicle 100 according to an embodiment of the presentinvention. The vehicle 100 has a powertrain 129 that includes an engine121 that is connected to a driveline 130 having an automatictransmission 124. It is to be understood that embodiments of the presentinvention are also suitable for use in vehicles with manualtransmissions, continuously variable transmissions or any other suitabletransmission.

In the embodiment of FIG. 1 the transmission 124 may be set to one of aplurality of transmission operating modes, being a park mode P, areverse mode R, a neutral mode N, a drive mode D or a sport mode S, bymeans of a transmission mode selector dial 124S. The selector dial 124Sprovides an output signal to a powertrain controller 11 in response towhich the powertrain controller 11 causes the transmission 124 tooperate in accordance with the selected transmission mode.

The driveline 130 is arranged to drive a pair of front vehicle wheels111, 112 by means of a front differential 137 and a pair of front driveshafts 118. The driveline 130 also comprises an auxiliary drivelineportion 131 arranged to drive a pair of rear wheels 114, 115 by means ofan auxiliary driveshaft or prop-shaft 132, a rear differential 135 and apair of rear driveshafts 139. The front wheels 111, 112 in combinationwith the front drive shafts 118 and front differential 137 may bereferred to as a front axle 136F. The rear wheels 114, 115 incombination with rear drive shafts 139 and rear differential 135 may bereferred to as a rear axle 136R.

The wheels 111, 112, 114, 115 each have a respective brake 111B, 112B,114B, 115B. Respective speed sensors 111S, 112S, 114S, 115S areassociated with each wheel 111, 112, 114, 115 of the vehicle 100. Thesensors 111S, 112S, 114S, 115S are mounted to a chassis 100C of thevehicle 100 and arranged to measure a speed of the corresponding wheel.

Embodiments of the invention are suitable for use with vehicles in whichthe transmission is arranged to drive only a pair of front wheels oronly a pair of rear wheels (i.e. front wheel drive vehicles or rearwheel drive vehicles) or selectable two wheel drive/four wheel drivevehicles. In the embodiment of FIG. 1 the transmission 124 is releasablyconnectable to the auxiliary driveline portion 131 by means of a powertransfer unit (PTU) 131P, allowing operation in a two wheel drive modeor a four wheel drive mode. It is to be understood that embodiments ofthe invention may be suitable for vehicles having more than four wheelsor where only two wheels are driven, for example two wheels of a threewheeled vehicle or four wheeled vehicle or a vehicle with more than fourwheels.

A control system for the vehicle engine 121 includes a centralcontroller 10, referred to as a vehicle control unit (VCU) 10, thepowertrain controller 11, a brake controller 13 and a steeringcontroller 170C. The brake controller 13 is an anti-lock braking system(ABS) controller 13 and forms part of a braking system 22 (FIG. 3). TheVCU 10 receives and outputs a plurality of signals to and from varioussensors and subsystems (not shown) provided on the vehicle. The VCU 10includes a low-speed progress (LSP) control system 12 shown in FIG. 3, astability control system (SCS) 14S, a traction control system (TCS) 14T,a cruise control system 16 and a Hill Descent Control (HDC) system 12HD.The SCS 14S improves stability of the vehicle 100 by detecting andmanaging loss of traction when cornering. When a reduction in steeringcontrol is detected, the SCS 14S is configured automatically to commanda brake controller 13 to apply one or more brakes 111B, 112B, 114B, 115Bof the vehicle 100 to help to steer the vehicle 100 in the direction theuser wishes to travel. If excessive wheel spin is detected, the TCS 14Sis configured to reduce wheel spin by application of brake force incombination with a reduction in powertrain drive torque. In theembodiment shown the SCS 14S and TCS 14T are implemented by the VCU 10.In some alternative embodiments the SCS 14S and/or TCS 14T may beimplemented by the brake controller 13. Further alternatively, the SCS14S and/or TCS 14T may be implemented by separate controllers.

Similarly, one or more of the controllers 10, 11, 13, 170C may beimplemented in software run on a respective one or more computingdevices such as one or more electronic control units (ECUs). In someembodiments two or more controllers may be implemented in software runon one or more common computing devices. Two or more controllers may beimplemented in software in the form of a combined software module, or aplurality of respective modules each implementing only one controller.

One or more computing devices may be configured to permit a plurality ofsoftware modules to be run on the same computing device withoutinterference between the modules. For example the computing devices maybe configured to allow the modules to run such that if execution ofsoftware code embodying one module terminates erroneously, or thecomputing device enters an unintended endless loop in respect of one ofthe modules, it does not affect execution by one or more computingdevices of software code comprised by a software module embodying thesecond controller.

It is to be understood that one or more of the controllers 10, 11, 13,170C may be configured to have substantially no single point failuremodes, i.e. one or more of the controllers may have dual or multipleredundancy. It is to be understood that robust partitioning technologiesare known for enabling redundancy to be introduced, such as technologiesenabling isolation of software modules being executed on a commoncomputing device. It is to be understood that the common computingdevice will typically comprise at least one microprocessor, optionally aplurality of processors, which may operate in parallel with one another.In some embodiments a monitor may be provided, the monitor beingoptionally implemented in software code and configured to raise an alertin the event a software module is determined to have malfunctioned.

The SCS 14S, TCS 14T, ABS controller 22C and HDC system 12HD provideoutputs indicative of, for example, SCS activity, TCS activity and ABSactivity including brake interventions on individual wheels and enginetorque requests from the VCU 10 to the engine 121, for example in theevent a wheel slip event occurs. Each of the aforementioned eventsindicate that a wheel slip event has occurred. Other vehicle sub-systemssuch as a roll stability control system or the like may also be present.

As noted above the vehicle 100 includes a cruise control system 16 whichis operable to automatically maintain vehicle speed at a selected speedwhen the vehicle is travelling at speeds in excess of 25 kph. The cruisecontrol system 16 is provided with a cruise control HMI (human machineinterface) 18 by which means the user can input a target vehicle speedto the cruise control system 16 in a known manner. In one embodiment ofthe invention, cruise control system input controls are mounted to asteering wheel 171 (FIG. 5). The cruise control system 16 may beswitched on by pressing a cruise control system selector button 176.When the cruise control system 16 is switched on, depression of a‘set-speed’ control 173 sets the current value of a cruise controlset-speed parameter, cruise_set-speed to the current vehicle speed.Depression of a ‘+’ button 174 allows the value of cruise_set-speed tobe increased whilst depression of a ‘−’ button 175 allows the value ofcruise_set-speed to be decreased. A resume button 173R is provided thatis operable to control the cruise control system 16 to resume speedcontrol at the instant value of cruise_set-speed following driverover-ride. It is to be understood that known on-highway cruise controlsystems including the present system 16 are configured so that, in theevent that the user depresses the brake or, in the case of vehicles witha manual transmission, a clutch pedal, the cruise control function iscancelled and the vehicle 100 reverts to a manual mode of operationwhich requires accelerator pedal input by a user in order to maintainvehicle speed. In addition, detection of a wheel slip event, as may beinitiated by a loss of traction, also has the effect of cancelling thecruise control function. Speed control by the system 16 is resumed ifthe driver subsequently depresses the resume button 173R.

The cruise control system 16 monitors vehicle speed and any deviationfrom the target vehicle speed is adjusted automatically so that thevehicle speed is maintained at a substantially constant value, typicallyin excess of 25 kph. In other words, the cruise control system isineffective at speeds lower than 25 kph. The cruise control HMI 18 mayalso be configured to provide an alert to the user about the status ofthe cruise control system 16 via a visual display of the HMI 18. In thepresent embodiment the cruise control system 16 is configured to allowthe value of cruise_set-speed to be set to any value in the range 25-150kph.

The LSP control system 12 also provides a speed-based control system forthe user which enables the user to select a very low target speed atwhich the vehicle can progress without any pedal inputs being requiredby the user. Low-speed speed control (or progress control) functionalityis not provided by the on-highway cruise control system 16 whichoperates only at speeds above 25 kph.

The LSP control system 12 is activated by means of a LSP control systemselector button 172 mounted on the steering wheel 171. The system 12 isoperable to apply selective powertrain, traction control and brakingactions to one or more wheels of the vehicle 100, collectively orindividually, to maintain the vehicle 100 at the desired speed.

The LSP control system 12 is configured to allow a user to input adesired value of set-speed parameter, LSP_set-speed to the LSP controlsystem 12 via a low-speed progress control HMI (LSP HMI) 20 (FIG. 1,FIG. 3) which shares certain input buttons 173-175 with the cruisecontrol system 16 and HDC control system 12HD. Provided the vehiclespeed is within the allowable range of operation of the LSP controlsystem (which is the range from 2 to 30 kph in the present embodimentalthough other ranges are also useful) the LSP control system 12controls vehicle speed in accordance with the value of LSP_set-speed.Unlike the cruise control system 16, the LSP control system 12 isconfigured to operate independently of the occurrence of a tractionevent. That is, the LSP control system 12 does not cancel speed controlupon detection of wheel slip. Rather, the LSP control system 12 activelymanages vehicle behaviour when slip is detected.

The LSP control HMI 20 is provided in the vehicle cabin so as to bereadily accessible to the user. The user of the vehicle 100 is able toinput to the LSP control system 12, via the LSP HMI 20, an indication ofthe speed at which the user desires the vehicle to travel (referred toas “the target speed”) by means of the ‘set-speed’ button 173 and the‘+’/‘−’ buttons 174, 175 in a similar manner to the cruise controlsystem 16. The LSP HMI 20 also includes a visual display upon whichinformation and guidance can be provided to the user about the status ofthe LSP control system 12.

The LSP control system 12 receives an input from the braking system 22of the vehicle indicative of the extent to which the user has appliedbraking by means of the brake pedal 163. The LSP control system 12 alsoreceives an input from an accelerator pedal 161 indicative of the extentto which the user has depressed the accelerator pedal 161. An input isalso provided to the LSP control system 12 from the transmission orgearbox 124. This input may include signals representative of, forexample, the speed of an output shaft of the gearbox 124, torqueconverter slip and a gear ratio request. Other inputs to the LSP controlsystem 12 include an input from the cruise control HMI 18 which isrepresentative of the status (ON/OFF) of the cruise control system 16,and an input from the LSP control HMI 20.

The HDC system 12HD is configured to limit vehicle speed when descendinga gradient. When the HDC system 12HD is active, the system 12HD controlsthe braking system 22 (via brake controller 13) in order to limitvehicle speed to a value corresponding to that of a HDC set-speedparameter HDC_set-speed which may be set by a user. The HDC set-speedmay also be referred to as an HDC target speed. Provided the user doesnot override the HDC system by depressing the accelerator pedal when theHDC system 12HD is active, the HDC system 12HD controls the brakingsystem 22 to prevent vehicle speed from exceeding the value ofHDC_set-speed. In the present embodiment the HDC system 12HD is notoperable to apply positive drive torque. Rather, the HDC system 12HD isonly operable to apply negative brake torque by means of the brakingsystem 22.

A HDC system HMI 20HD is provided by means of which a user may controlthe HDC system 12HD, including setting the value of HDC_set-speed. AnHDC system selector button 177 is provided on the steering wheel 171 bymeans of which a user may activate the HDC system 12HD to controlvehicle speed.

As noted above, the HDC system 12HD is operable to allow a user to set avalue of HDC set-speed parameter HDC_set-speed and to adjust the valueof HDC_set-speed using the same controls as the cruise control system 16and LSP control system 12. Thus, in the present embodiment, when the HDCsystem 12HD is controlling vehicle speed, the HDC system set-speed maybe increased, decreased or set to an instant speed of the vehicle in asimilar manner to the set-speed of the cruise control system 16 and LSPcontrol system 12, using the same control buttons 173, 173R, 174, 175.The HDC system 12HD is operable to allow the value of HDC_set-speed tobe set to any value in the range from 2-30 kph.

If the HDC system 12HD is selected when the vehicle 100 is travelling ata speed of 50 kph or less and no other speed control system is inoperation, the HDC system 12HD sets the value of HDC_set-speed to avalue selected from a look-up table. The value output by the look-uptable is determined in dependence on the identity of the currentlyselected transmission gear, the currently selected PTU gear ratio(Hi/LO) and the currently selected driving mode. The HDC system 12HDthen applies the powertrain 129 and/or braking system 22 to slow thevehicle 100 to the HDC system set-speed provided the driver does notoverride the HDC system 12HD by depressing the accelerator pedal 161.The HDC system 12HD is configured to slow the vehicle 100 to theset-speed value at a deceleration rate not exceeding a maximum allowablerate although as noted elsewhere the HDC system 12HD is not able tocause positive drive torque to be applied by the powertrain 129 in orderto reduce a rate of deceleration of the vehicle 100. The rate is set at1.25 ms-2 in the present embodiment, however other values are alsouseful. If the user subsequently presses the ‘set-speed’ button 173 theHDC system 12HD sets the value of HDC_set-speed to the instant vehiclespeed provided the instant speed is 30 kph or less. If the HDC system12HD is selected when the vehicle 100 is travelling at a speed exceeding50 kph, the HDC system 12HD ignores the request and provides anindication to the user that the request has been ignored.

In the present embodiment the vehicle 100 is configured to assume one ofa plurality of power modes PM at a given moment in time. In each powermode the vehicle 100 may be operable to allow a predetermined set of oneor more operations to be performed. For example, the vehicle 100 mayallow a predetermined one or more vehicle subsystems such as aninfotainment system, a windscreen demist subsystem and a windscreenwiper control system to be activated only in a respective one or morepredetermined power modes. In one or more of the power modes the vehicle100 may be configured to inhibit one or more operations, such as turningon of the infotainment system.

The identity of the power mode in which the vehicle 100 is to operate ata given moment in time is transmitted to each controller 10, 11, 12, 13,14, 16, 12HD, of the vehicle 100 by the central controller 10. Thecontrollers respond by assuming a predetermined state associated withthat power mode and that controller. In the present embodiment eachcontroller may assume an ON state in which the controller is configuredto execute computer program code associated with that controller, and anOFF state in which supply of power to the controller is terminated. Inthe present embodiment, the central controller 10 is also operable toassume a quiescent state. The quiescent state is assumed by the centralcontroller 10 when the vehicle is in power mode PM0 and the controller10 has confirmed that the other controllers 11, 12, 13, 14, 16, 12HDhave successfully assumed the OFF state following receipt of the commandto assume power mode PM0.

In the present embodiment the vehicle 100 is provided with a known keyfob 190 (FIG. 6) that has a radio frequency identification device (RFID)190R embedded therein. The key fob 190 has first and second controlbuttons 191, 192. The key fob 100 is configured to generate a respectiveelectromagnetic signal in response to depression of the first or secondcontrol buttons 191, 192. The central controller 10 detects theelectromagnetic signal by means of a receiver module forming part of thecontroller 10 and triggers locking or unlocking of door locks 182L ofthe vehicle 100. Each door 100D of the vehicle 100 is provided with arespective door lock 182L as shown in FIG. 2.

Pressing of the first control button 191 generates a door unlock signal,which triggers unlocking of the door locks 182L, whilst pressing of thesecond control button 192 triggers a door lock signal, which triggerslocking of the door locks 182L.

When the controller 10 is in the quiescent state, consumption of powerby the central controller 10 is reduced and the controller 10 monitorsreceipt of a door unlock signal from the key fob 190. It is to beunderstood that in some embodiments one or more vehicle controllers maybe configured to remain in the ON or quiescent state, to allow one ormore essential functions to be performed, when the vehicle is in powermode PM0. For example in vehicles fitted with an intruder alarm systeman intruder alarm controller may be permitted to remain in the ON or aquiescent state pending detection of an intrusion. Upon detection of anintrusion the intruder alarm controller may cause the central controller10 to assume the ON state if it is not already in that state.

The central controller 10 is also configured to transmit a radiofrequency (RF) ‘interrogation’ signal that causes the RFID device 190Rof the key fob 190 to generate an RF ‘acknowledgement’ signal inresponse to receipt of the interrogation signal. In the presentembodiment the RFID device 190R is a passive device, not requiringbattery power in order to generate the acknowledgement signal. Thecontroller 10 is configured to detect the acknowledgment signaltransmitted by the RFID device 190R provided the RFID device 190R iswithin range. By the term ‘within range’ is meant that the RFID device190R or fob 190 is sufficiently close to the controller 10 to receivethe interrogation signal and generate an acknowledgement signal that isdetectable by the controller 10.

The vehicle 100 is also provided with a start/stop button 181. Thestart/stop button 181 is configured to transmit a signal to the centralcontroller 10 when pressed in order to trigger an engine startoperation, provided certain predetermined conditions are met. Inresponse to pressing of the start/stop button 181 the central controller10 causes the vehicle 100 to be placed in a condition in which if thetransmission 124 is subsequently placed in the forward driving mode D orreverse driving mode R, the vehicle 100 may be driven by depressingaccelerator pedal 161. In the present embodiment, the central controller10 is configured to perform a pre-start verification operation beforecommanding the powertrain controller 11 to trigger an engine startoperation. In performing the pre-start verification operation thecontroller 10 verifies (a) that the vehicle 100 is in a predeterminedpower mode as described in more detail below, (b) that the controller 10is receiving an acknowledgement signal from the key fob 190 in responseto transmission of the interrogation signal by the controller 10, and(c) that the transmission 124 is in either the park P or neutral Nmodes. Thus, the controller 10 requires that the RFID device 190R iswithin range of the controller 10 before permitting an engine start. Ifany of conditions (a) to (c) are not met the controller causes thevehicle 100 to remain in its current power mode.

It is to be understood that the central controller 10 is configured tocause the vehicle 100 to assume a predetermined one of a plurality ofpower modes in dependence at least in part on actuation of a controlbutton 191, 192 of the key fob 190 and actuation of the start/stopbutton 181. In some embodiments the vehicle 100 may be configured suchthat the central controller 10 responds to voice commands from a user inaddition to or instead of signals received from the key fob 190.

The various power modes in which the vehicle 100 of the embodiment ofFIG. 1 may be operated will now be described. As noted above, the keyfob 190 is operable to cause the door locks 182L of the vehicle 100 tobe locked and unlocked. When the doors 100D of the vehicle 100 (FIG. 2)are closed and the locks 182L are in the locked condition, the vehicle100 assumes power mode PM0.

If the first button 191 of the key fob 190 is subsequently actuated, thecontroller 10 causes the door locks 182L to assume the unlockedcondition. Once the door locks 182L are in the unlocked condition andthe controller 10 detects the acknowledgement signal from the key fob190, the controller 10 causes the vehicle 100 to assume power mode PM4.In power mode PM4 the controller 10 permits a predetermined number ofelectrical systems to become active, including an infotainment system.Power mode PM4 may also be referred to as a convenience mode oraccessory mode. If a user subsequently presses the second button 192 ofthe key fob 190, the controller 10 causes the vehicle 100 to revert topower mode PM0.

If, whilst the vehicle is in power mode PM4 a user presses the starterbutton 181 and maintains the button 181 in a depressed condition, thecontroller 10 performs the pre-start verification operation describedabove. Provided conditions (a) to (c) of the pre-start verificationoperation are met, the controller 10 places the vehicle 100 in powermode PM6. When the vehicle 100 is in power mode PM6 the powertraincontroller 11 is permitted to activate a starter device. In the presentembodiment the starter device is a starter motor 121M. The powertraincontroller 11 is then commanded to perform an engine start operation inwhich the engine 121 is cranked by means of the starter motor 121M tocause the engine 121 to start. Once the controller 10 determines thatthe engine 121 is running, the controller 10 places the vehicle 100 inpower mode PM7.

In power mode PM6 the controller 10 disables certain non-criticalelectrical systems including the infotainment system. This is at leastin part so as to reduce the magnitude of the electrical load on abattery 100B of the vehicle during cranking in order to permit anincrease in the amount of electrical current available for enginestarting. Isolation of non-critical electrical systems also reduces arisk of damage to the systems when a relatively large current drain isplaced on the battery 100B by the starter motor 121M.

If whilst the vehicle is in power mode PM7, with the engine 121 running,a user again actuates the start/stop button 181, the controller 10causes the powertrain controller 11 to switch off the engine 121 and thecontroller 10 causes the vehicle 100 to transition to power mode PM4. Auser may then cause the vehicle to assume power mode PM0 by pressing thefirst button 191 of the key fob 190 provided each of the doors 100D isclosed. It is to be understood that in some embodiments the user maytrigger assumption of power mode PM0 whilst remaining in the vehicle 100and locking the doors 181 by means of the key fob 190. In someembodiments the vehicle 100 may be configured to assume power mode PM0regardless of whether the controller is receiving the acknowledgementsignal from the key fob 190. Other arrangements are also useful.

It is to be understood that assumption of power mode PM0 by the vehicle100 may be referred to as ‘key off’, whilst assumption of power mode PM4from power mode PM0 may be referred to as ‘key on’. A sequence oftransitions of the vehicle from power mode PM0 to PM4, and back to powermode PM0, optionally including one or more transitions to power mode PM6and power mode PM7 prior to assumption of power mode PM0, may bereferred to as a ‘key cycle’. Thus a key cycle begins and ends with thevehicle 100 in power mode PM0. In some embodiments, assumption of powermode PM6 or PM7 from power mode PM0 may be required in order to completea key cycle, starting with power mode PM0.

It is to be understood that the VCU 10 is configured to implement aknown Terrain Response (TR) (RTM) System of the kind described above inwhich the VCU 10 controls settings of one or more vehicle systems orsub-systems such as the powertrain controller 11 in dependence on aselected driving mode. The driving mode may be selected by a user bymeans of a driving mode selector 141S (FIG. 1). The driving modes mayalso be referred to as terrain modes, terrain response modes, or controlmodes. In the embodiment of FIG. 1 four driving modes are provided: an‘on-highway’ driving mode suitable for driving on a relatively hard,smooth driving surface where a relatively high surface coefficient offriction exists between the driving surface and wheels of the vehicle; a‘sand’ driving mode suitable for driving over sandy terrain; a ‘grass,gravel or snow’ driving mode suitable for driving over grass, gravel orsnow, a ‘rock crawl’ driving mode suitable for driving slowly over arocky surface; and a ‘mud and ruts’ driving mode suitable for driving inmuddy, rutted terrain. Other driving modes may be provided in additionor instead.

In the present embodiment, at any given moment in time the LSP controlsystem 12 is in one of a plurality of allowable ‘on’ modes (alsoreferred to as conditions or states) selected from amongst an active orfull function (FF) mode, a descent control (DC) mode, also referred toas an intermediate mode and a standby mode. The LSP control system mayalso assume an ‘off’ mode or condition. The active mode, DC mode andstandby mode may be considered to be different ‘on’ modes or conditionsof the vehicle, i.e. different modes in which the LSP control system isin an ‘on’ mode or condition as opposed to an ‘off’ mode or condition.In the off condition the LSP control system 12 only responds to pressingof the LSP selector button 172, which causes the LSP control system 12to assume the on condition and the DC mode. When the LSP control system12 assumes the on mode from the off mode in response to pressing of theLSP selector button, the value of LSP_set-speed is set to the instantspeed of the vehicle 100 provided it is in the allowable range of speedsfor operation of the LSP control system 12. If the vehicle speed 100 isabove this range the value of LSP_set-speed is set to the highestallowable speed for operation of the LSP control system 12, i.e. 30 kph.

In the active or full function mode, the LSP control system 12 activelymanages vehicle speed in accordance with the value of LSP set-speed,LSP_set-speed, by causing the application of positive powertrain drivetorque to one or more driving wheels or negative braking system torqueto one or more braked wheels.

In the DC mode the LSP control system 12 operates in a similar manner tothat in which it operates when in the active mode except that the LSPcontrol system 12 is prevented from commanding the application ofpositive drive torque by means of the powertrain 129. Rather, onlybraking torque may be applied, by means of the braking system 22 and/orpowertrain 129. The LSP control system 12 is configured to increase ordecrease the amount of brake torque applied to one or more wheels inorder to cause the vehicle to maintain the LSP set-speed to the extentpossible without application of positive drive torque. It is to beunderstood that, in the present embodiment, operation of the LSP controlsystem 12 in the DC mode is very similar to operation of the HDC system12HD, except that the LSP control system 12 continues to employ the LSPcontrol system 12 set-speed value LSP_set-speed rather than the HDCcontrol system set-speed value HDC_set-speed.

In the standby mode, the LSP control system 12 is unable to causeapplication of positive drive torque or negative brake torque to awheel.

As noted above, in the ‘off’ mode the LSP control system 12 is notresponsive to any LSP input controls except the LSP control systemselector button 172. Pressing of the LSP control system selector button172 when the system 12 is in the off mode causes the system 12 to assumethe ‘on’ condition and the DC mode.

When the LSP control system 12 is initially switched on by means of theLSP selector button 172, the LSP control system 12 assumes the DC mode.

If whilst in DC mode the ‘set +’ button 174 is pressed, the LSP controlsystem 12 sets the value of LSP_set-speed to the instant value ofvehicle speed according to vehicle speed signal 36 (FIG. 4, discussed inmore detail below) and assumes the active mode. If the vehicle speed isabove 30 kph, being the maximum allowable value of LSP_set-speed, theLSP control system 12 remains in the DC mode and ignores the request toassume the active mode. A signal may be provided to the driverindicating that the LSP control system 12 cannot be activated due to thevehicle speed exceeding the maximum allowable value of LSP_set-speed.The signal may be provided by means of a text message provided on theLSP control HMI 18, by means of an indicator lamp, an audible alert orany other suitable means.

If the resume button 173R is depressed whilst in the DC mode, the LSPcontrol system assumes the active mode and causes the vehicle to operatein accordance with the stored value of LSP_set-speed provided thevehicle speed does not exceed 30 kph.

If vehicle speed is above 30 kph but less than or substantially equal to50 kph when the resume button 173R is pressed the LSP control system 12remains in the DC mode until vehicle speed falls below 30 kph. In the DCmode, provided the driver does not depress the accelerator pedal 161 theLSP control system 12 deploys the braking system 22 to slow the vehicle100 to a value of set-speed corresponding to the value of parameterLSP_set-speed. Once the vehicle speed falls to 30 kph or below, the LSPcontrol system 12 assumes the active mode in which it is operable tocause a required amount of positive powertrain drive torque to beapplied to one or more wheels via the powertrain 129, as well asnegative torque via the powertrain 129 (via engine braking) and braketorque via the braking system 22 in order to control the vehicle inaccordance with the LSP_set-speed value. The LSP control system 12 maygenerate a virtual accelerator pedal signal in order to cause thepowertrain 129 to develop a required amount of powertrain torque in someembodiments. The virtual accelerator pedal signal may correspond to thatwhich would be generated by an accelerator pedal controller in responseto depression of the accelerator pedal 161 by an amount corresponding tothe amount of powertrain torque required at a given moment in time. Theaccelerator pedal controller may form part of a powertrain controller 11although other arrangements are also useful.

With the LSP control system 12 in the active mode, the user may increaseor decrease the value of LSP_set-speed by means of the ‘+’ and ‘−’buttons 174, 175. In addition, the user may optionally also increase ordecrease the value of LSP_set-speed by lightly pressing the acceleratoror brake pedals 161, 163 respectively. In some embodiments, with the LSPcontrol system 12 in the active mode the ‘+’ and ‘−’ buttons 174, 175may be disabled such that adjustment of the value of LSP_set-speed canonly be made by means of the accelerator and brake pedals 161, 163. Thislatter feature may prevent unintentional changes in set-speed fromoccurring, for example due to accidental pressing of one of the ‘+’ or‘−’ buttons 174, 175. Accidental pressing may occur for example whennegotiating difficult terrain where relatively large and frequentchanges in steering angle may be required. Other arrangements are alsouseful.

It is to be understood that in the present embodiment the LSP controlsystem 12 is operable to cause the vehicle to travel in accordance witha value of set-speed in the range from 2-30 kph whilst the cruisecontrol system is operable to cause the vehicle to travel in accordancewith a value of set-speed in the range from 25-150 kph although othervalues are also useful, such as 30-120 kph or any other suitable rangeof values.

It is to be understood that if the LSP control system 12 is in theactive mode, operation of the cruise control system 16 is inhibited. Thetwo speed control systems 12, 16 therefore operate independently of oneanother, so that only one can be operable at any one time.

In some embodiments, the cruise control HMI 18 and the LSP control HMI20 may be configured within the same hardware so that, for example, thespeed selection is input via the same hardware, with one or moreseparate switches being provided to switch between the LSP control HMI20 and the cruise control HMI 18.

When in the active mode, the LSP control system 12 is configured tocommand application of positive powertrain torque and negative braketorque, as required, by transmitting a request for (positive) drivetorque in the form of a powertrain torque signal and/or a request for(negative) brake torque in the form of a brake torque signal to thebrake controller 13. The brake controller 13 arbitrates any demand forpositive powertrain torque, determining whether the request for positivepowertrain torque is allowable. If a request for positive powertraintorque is allowable the brake controller 13 issues the request to thepowertrain controller 11. In some embodiments, the request for braketorque may correspond to an amount of brake torque (or brake fluidpressure) to be developed by the braking system 22. In some alternativeembodiments the request for brake torque may be for an amount ofnegative torque to be applied to one or more wheels. The brakecontroller 13 may in some embodiments determined whether the requestednegative torque is to be supplied by means of powertrain braking alone,for example engine overrun braking, by means of powertrain braking andbrake torque developed by the braking system 22, or by means of thebraking system 22 alone. In some embodiments the brake controller 13 orLSP control system 12 may be configured to cause a required amount ofnet negative torque to be applied to one or more wheels by applyingnegative torque by means of the braking system 22 against positive drivetorque generated by the powertrain 129. Application of positive drivetorque generated by means of the powertrain 129 against negative braketorque generated by means of the braking system 22 may be made in orderto reduce wheel flare when driving on surfaces of relatively low surfacecoefficient of friction such as during off-road driving. By wheel flareis meant excessive wheel slip as a result of the application of excesspositive net torque to a wheel.

In the present embodiment the brake controller 13 also receives from theLSP control system 12 a signal S_mode indicative of the mode in whichthe LSP control system 12 is operating, i.e. whether the LSP controlsystem 12 is operating in the active mode, DC mode, standby mode or offmode.

If the brake controller 13 receives a signal S_mode indicating that theLSP control system 12 is operating in the DC mode, standby mode or offmode, the brake controller 13 sets a powertrain torque request inhibitflag in a memory thereof. The powertrain torque request inhibit flagindicates that positive torque requests to the powertrain controller 11from the brake controller 13 in response to positive torque requestsfrom the LSP control system 12 are forbidden. Accordingly, if a requestfor positive powertrain torque is received by the brake controller 13from the LSP control system 12 whilst the LSP control system 12 isoperating in the DC mode, standby mode or off mode, the positive torquerequest is ignored by the brake controller 13.

In some embodiments, the powertrain controller 11 is also provided withsignal S_mode indicating the mode in which the LSP control system 12 isoperating. If the LSP control system 12 is operating in a mode otherthan the active mode, such as the DC mode, standby mode or off mode,positive powertrain torque requests received as a consequence of acommand from the LSP control system 12 are ignored by the powertraincontroller 11.

In some embodiments, if the powertrain controller 11 receives a requestfor positive powertrain torque from the brake controller 13 as aconsequence of a command from the LSP control system 12 and the requestis received more than a predetermined period after the LSP controlsystem 12 has transitioned to a mode other than the active mode, thepowertrain controller 11 causes the LSP control system 12 to assume adisabled off mode. In the disabled off mode the LSP control system 12 iseffectively locked into the off condition or mode for the remainder ofthe current key cycle and the LSP control system 12 does not assume theDC mode in response to pressing of the LSP selector button 172. Thepredetermined period may be any suitable period such as 50 ms, 100 ms,500 ms, 1000 ms or any other suitable period. The period may be set to avalue such that any delay in receipt of a positive torque request issuedby the LSP control system 12 immediately prior to a transition from theactive mode to a mode other than the active mode (and in which positivetorque requests are not permitted) that is consistent with normal systemoperation will not trigger a transition to the disabled off mode.However, the powertrain controller 11 is configured such that anyrequest for positive powertrain torque received by the powertraincontroller 11 as a consequence of a request issued by the LSP controlsystem 12 after assuming a mode other than the active mode (and in whichpositive torque requests are not permitted) will trigger a transition tothe disabled off mode.

It is to be understood that other arrangements may also be useful. Forexample, in some embodiments, in the disabled off mode the LSP controlsystem 12 may be configured not to respond to the LSP selector button172 by assuming the DC mode until after the vehicle has transitionedfrom power mode PM7 to power mode PM4. As described above, a transitionfrom power mode PM7 to power mode PM4 may be accomplished by depressingthe start/stop button 124S. When the vehicle 100 is subsequentlyrestarted and assumes power mode PM7, the LSP control system 12 may bepermitted to assume operation in the active mode as required.

It is to be understood that some vehicles may be provided with knownautomatic engine stop/start functionality. In vehicles with thisfunctionality, the powertrain controller 11 is configured to commandstopping and starting of the engine 121 according to a stop/startcontrol methodology when the vehicle 100 is being held stationary bymeans of brake pedal 163 with the transmission in the drive mode D. Theprocess of automatically commanding stopping and restarting of theengine 121 may be referred to as an automatic stop/start cycle. Invehicles having automatic engine stop/start functionality, thecontroller 10 may be configured to cause the vehicle 100 to assume apower mode PM6A when the engine 121 is stopped during a stop/startcycle. Power mode PM6A is similar to power mode 6, except that disablingof certain vehicle systems such as the infotainment system is notperformed when in power mode PM6A. In power mode PM6A, the powertraincontroller 11 is configured to restart the engine 121 upon receipt of asignal indicating a user has released the brake pedal 163. It is to beunderstood that in some embodiments, a vehicle 100 may be configured torequire an engine restart before the LSP control system 12 may exit theDC fault mode but an engine restart as part of an automatic stop/startcycle may be configured not to qualify as an engine restart permittingthe system 12 to exit the DC fault mode. In some embodiments therefore,a transition from power mode PM7 to power mode PM6A and back to powermode PM7 does not permit the LSP control system 12 to exit the disabledoff mode.

In some embodiments the LSP control system 12 may be configured suchthat it can assume one of a number of different further modes such as:

(i) DC fault mode

(ii) DC fault mode fade-out mode

(iii) DC mode fade-out mode

(iv) Active standby mode

(v) DC standby mode

The DC fault mode corresponds to the DC mode except that if the DC faultmode is assumed by the LSP control system 12, the LSP control system 12is unable subsequently to assume the active mode for the remainder ofthe current key cycle. Thus, when the next key-on procedure isperformed, following the next key-off procedure, the LSP control system12 is permitted to assume the active mode when required. The vehicle 100may be configured wherein the LSP control system 12 may assume the DCfault mode if a fault is detected indicating that the LSP control system12 should not be permitted to request positive powertrain drive torquebut where it is determined that it may be desirable for the benefits ofDC mode to be enjoyed. Thus a transition from active mode to DC faultmode may be preferable to a transition to the off mode, particularlywhen negotiating off road conditions, in the event of a relatively minorfault in respect of the LSP control system 12.

In some embodiments, if a transition to DC fault mode occurs with morethan a predetermined frequency, the LSP control system 12 may becomelatched in the DC fault mode until a reset procedure is performedrequiring action other than a key-off and subsequent key-on procedure inorder to permit the active mode to be assumed again. In someembodiments, the LSP control system 12 may require a predetermined codeto be provided to it. In some embodiments, the LSP control system 12 maybe configured to receive the code via a computing device external to thevehicle 100 that temporarily communicates with the LSP control system 12in order to provide the code. The computing device may be a devicemaintained by a vehicle servicing organisation such as a dealercertified by a manufacturer of the vehicle 100. The computing device maybe in the form of a laptop or other computing device, and be configuredto communicate wirelessly with the LSP control system 12 or via a wiredconnection.

The predetermined frequency may be defined in terms of a predeterminednumber of occurrences in a predetermined number of key cycles, or apredetermined distance driven, or be time based such as a predeterminednumber of occurrences in a predetermined period in which the vehicle isin power mode 7 (or power mode 6A in addition to power mode 7, in thecase of a vehicle with stop/start functionality) over one or more keycycles, or a predetermined number of occurrences in a given calendarperiod, such as a day, a week, a month or any other suitable frequency.

The DC fault mode fade-out mode is a mode assumed by the LSP controlsystem 12 when transitioning from the DC fault mode to an off mode suchas disabled off, unless an immediate (‘binary’) transition to an offmode is required in which case the DC fault mode fade-out mode is notassumed. Thus, under certain conditions, rather than abruptly terminatecommanding application of brake torque by means of the braking system 22when ceasing operation in the DC fault mode and transitioning to an offmode such as ‘off’ or ‘disabled off’, the LSP control system 12gradually fades out the application of any brake torque applied by thebraking system 22 as a consequence of being in the DC fault mode, beforeassuming the off or disabled off mode. This is at least in part so as toallow a driver time to adapt to driving without the system 12 applyingbrake torque automatically.

Similarly, if the LSP control system 12 transitions from the DC mode toa mode in which the LSP control system 12 is unable to commandapplication of brake torque such as the standby mode, off mode ordisabled off mode, the LSP control system 12 may assume the DC fade-outmode as an intermediate mode. In the DC fade-out mode, like the DC faultmode fade-out mode, the LSP control system 12 gradually reduces theamount of any brake torque commanded by the LSP control system 12,before assuming the target mode such as standby mode, off mode ordisabled off mode.

The active standby mode is a mode assumed by the LSP control system 12from the active mode if the driver over-rides the LSP control system 12by depressing the accelerator pedal 161 to increase vehicle speed. Ifthe driver subsequently releases the accelerator pedal with vehiclespeed within the allowable range for the LSP control system 12 tooperate in the active mode (i.e. a speed in the range 2-30 kph), the LSPcontrol system 12 resumes operation in the active mode.

The DC standby mode is a mode assumed by the LSP control system 12 ifwhilst operating in the DC mode the driver over-rides the LSP controlsystem 12 by depressing the accelerator pedal 161. If the driversubsequently releases the accelerator pedal, then when vehicle speed iswithin the allowable range for the LSP control system 12 to operate inthe DC mode (i.e. a speed in the range 2-30 kph), the LSP control system12 resumes operation in the DC mode. Other arrangements are also useful.In some embodiments the LSP control system 12 may be configured toassume DC mode from the DC standby mode and cause application of braketorque to slow the vehicle 100 when a driver releases the acceleratorpedal 161 even at speeds above 30 kph. In some embodiments the LSPcontrol system 12 may be configured to cause application of brake torqueat speeds of up to 50 kph, 80 kph or any other suitable speed in orderto cause vehicle speed to reduce to the LSP target speed LSP_set-speed.The LSP control system 12 may be configured to take into accountnegative torque applied by a powertrain due for example to engineover-run braking in determining an amount of brake torque required inorder to cause a vehicle to slow at a desired rate. The LSP controlsystem 12 may be configured to cause a vehicle to slow at a desired rateaccording to a predetermined deceleration profile.

In some embodiments, if the powertrain controller 11 receives a requestfor positive powertrain torque from the brake controller 13 as aconsequence of a command from the LSP control system 12 and the LSPcontrol system 12 is in the DC mode, the powertrain controller 11 causesthe LSP control system 12 to assume the DC fault mode if the positivetorque request is received more than a predetermined period after theLSP control system 12 has transitioned to the DC mode. As noted above,in the DC fault mode the LSP control system 12 is permitted to causeapplication of brake torque by the braking system 22 to control vehiclespeed but is prevented from assuming the active or FF mode for theremainder of the current key cycle. In these circumstances, the LSPcontrol system 12 assumes the DC fault mode substantially immediatelywith no requirement to blend the transition between the DC mode and DCfault mode.

As noted above, the predetermined period may be any suitable period suchas 50 ms, 100 ms, 500 ms, 1000 ms or any other suitable period. Theperiod may be set to a value such that any inherent system delay inreceipt by the powertrain controller 11 of a torque request from thebrake controller 13 as a consequence of a request issued by the LSPcontrol system 12 prior to a transition from the active mode to the DCmode will not trigger a transition to the DC fault mode. It is to beunderstood that by inherent system delay is meant a delay in signalreceipt that occurs during normal operation, for example due to arequirement to synchronise timing signals, or to transmit commands fromthe LSP control system 12 to the powertrain controller 11 atpredetermined intervals as part of an inter-controller communicationsprotocol.

In some embodiments, if the powertrain controller 11 receives a requestfor positive powertrain torque from the brake controller 13 as aconsequence of a command from the LSP control system 12 and the LSPcontrol system 12 is in the DC fault mode or DC fault mode fade out modeonly, the powertrain controller 11 causes the LSP control system 12 toassume the disabled off mode if the positive torque request is receivedmore than a predetermined period after the LSP control system 12 hastransitioned to the DC fault mode or DC fault mode fade out mode. In thepresent embodiment the predetermined period is a period of 500 ms.However the predetermined period may be any suitable period such as 50ms, 100 ms, 1000 ms or any other suitable period. The LSP control system12 may be configured substantially abruptly to terminate application ofany negative (brake) torque requested by the LSP control system 12 whenthe transition to the disabled off mode is commanded even if the system12 is in the processes of fading out any negative brake torque that isbeing applied as a result of a request issued by the LSP control system12

In some embodiments, in addition or instead, if the powertraincontroller 11 receives a request for positive powertrain torque from thebrake controller 13 as a consequence of a command from the LSP controlsystem 12 and the signal S_mode indicates that the LSP control system 12is in the DC mode, DC standby mode, DC mode fade-out mode or activestandby mode, the powertrain controller 11 causes the LSP control system12 to assume the disabled off mode if the positive torque request isreceived over a sustained period of more than a predetermined period. Inthe present embodiment the predetermined period is substantially 500 ms.However the predetermined period may be any suitable period such as 100ms, 1000 ms or any other suitable period. The LSP control system 12 isconfigured gradually to cause fade-out of any negative (brake) torquebeing applied as a consequence of a command from the LSP control system12 when the transition to the disabled off mode is commanded. Thefade-out of brake torque may be accomplished by assuming the DC modefade-out mode or DC fault mode fade-out mode if they have not alreadybeen assumed.

In some embodiments, the LSP control system 12 is caused to assume thedisabled off mode if the powertrain controller 11 receives a request forpositive powertrain torque from the brake controller 13 as a consequenceof a command from the LSP control system 12 and signal S_mode indicatesthat the LSP control system 12 is in the DC fault mode or DC fault modefade-out mode, as well as when the signal indicates the LSP controlsystem 12 is in the DC mode, DC standby mode, DC mode fade-out mode oractive standby mode.

It is to be understood that in some embodiments, instead of graduallyfading out negative brake torque, the LSP control system 12 may beconfigured to abruptly terminate application of any negative braketorque as a consequence of a command by the LSP control system 12. Thus,if a request for positive powertrain torque is received over a sustainedperiod of more than the predetermined period when the LSP control system12 is in the DC mode, DC standby mode, DC fault mode, DC mode fade-outmode, DC fault mode fade-out mode or active standby mode the system mayabruptly terminate application of brake torque caused by the LSP controlsystem 12. It is to be understood that the braking system 12 continuesto respond to driver brake commands via the brake pedal 163.

It is to be understood that in the present embodiment if a driverswitches off the LSP control system 12 manually, the LSP control system12 is configured gradually to cause fade-out of any negative (brake)torque being applied as a consequence of a command from the LSP controlsystem 12. This feature has the advantage that vehicle composure may beenhanced.

FIG. 4 illustrates the means by which vehicle speed is controlled in theLSP control system 12. As described above, a speed selected by a user(set-speed) is input to the LSP control system 12 via the LSP controlHMI 20. A vehicle speed calculator 34 provides a vehicle speed signal 36indicative of vehicle speed to the LSP control system 12. The speedcalculator 34 determines vehicle speed based on wheel speed signalsprovided by wheel speed sensors 111S, 112S, 114S, 115S. The LSP controlsystem 12 includes a comparator 28 which compares the LSP control systemset-speed LSP_set-speed 38 (also referred to as a ‘target speed’ 38)selected by the user with the measured speed 36 and provides an outputsignal 30 indicative of the comparison. The output signal 30 is providedto an evaluator unit 40 of the VCU 10 which interprets the output signal30 as either a demand for additional torque to be applied to the vehiclewheels 111-115, or for a reduction in torque applied to the vehiclewheels 111-115, depending on whether the vehicle speed needs to beincreased or decreased to maintain the speed LSP_set-speed. An increasein torque is generally accomplished by increasing the amount ofpowertrain torque delivered to a given position of the powertrain, forexample an engine output shaft, a wheel or any other suitable location.A decrease in torque at a given wheel to a value that is less positiveor more negative may be accomplished by decreasing the amount of anypositive powertrain torque delivered to a wheel, by increasing theamount of any negative powertrain torque delivered to a wheel, forexample by reducing an amount of air and/or fuel supplied to an engine121, and/or by increasing a braking force on a wheel. It is to beunderstood that in some embodiments in which a powertrain 129 has one ormore electric machines operable as a generator, negative torque may beapplied by the powertrain 129 to one or more wheels by means of theelectric machine. As noted above negative torque may also be applied bymeans of engine braking in some circumstances, depending at least inpart on the speed at which the vehicle 100 is moving. If one or moreelectric machines are provided that are operable as propulsion motors,positive drive torque may be applied by means of the one or moreelectric machines.

An output 42 from the evaluator unit 40 is provided to the brakecontroller 13. The brake controller 13 in turn controls a net torqueapplied to the vehicle wheels 111-115 by commanding application of braketorque via the brakes 111B, 112B, 114B, 115B and/or positive drivetorque by commanding powertrain controller 11 to deliver a requiredamount of powertrain torque. The net torque may be increased ordecreased depending on whether the evaluator unit 40 demands positive ornegative torque. In order to cause application of the necessary positiveor negative torque to the wheels, the brake controller 13 may commandthat positive or negative torque is applied to the vehicle wheels by thepowertrain 129 and/or that a braking force is applied to the vehiclewheels by the braking system 22, either or both of which may be used toimplement the change in torque that is necessary to attain and maintaina required vehicle speed. In the illustrated embodiment the torque isapplied to the vehicle wheels individually so as to maintain the vehicle100 at the required speed, but in another embodiment torque may beapplied to the wheels collectively to maintain the required speed. Insome embodiments, the powertrain controller 11 may be operable tocontrol an amount of torque applied to one or more wheels at least inpart by controlling a driveline component such as a rear drive unit,front drive unit, differential or any other suitable component. Forexample, one or more components of the driveline 130 may include one ormore clutches operable to allow an amount of torque applied to wheels ofa given axle to be controlled independently of the torque applied towheels of another axle, and/or the amount of torque applied to one ormore individual wheels to be controlled independently of other wheels.Other arrangements are also useful.

Where a powertrain 129 includes one or more electric machines, forexample one or more propulsion motors and/or generators, the powertraincontroller 11 may be operable to modulate or control the amount oftorque applied to one or more wheels at least in part by means of theone or more electric machines.

The LSP control system 12 also receives a signal 48 indicative of awheel slip event having occurred. This may be the same signal 48 that issupplied to the on-highway cruise control system 16 of the vehicle, andwhich in the case of the latter triggers an override or inhibit mode ofoperation of the on-highway cruise control system 16 so that automaticcontrol of vehicle speed by the on-highway cruise control system 16 issuspended or cancelled. However, the LSP control system 12 is notarranged to cancel or suspend operation in dependence on receipt of awheel slip signal 48 indicative of wheel slip. Rather, the system 12 isarranged to monitor and subsequently manage wheel slip so as to reducedriver workload. During a slip event, the LSP control system 12continues to compare the measured vehicle speed with the value ofLSP_set-speed, and continues to control automatically the torque appliedto the vehicle wheels so as to maintain vehicle speed at the selectedvalue. It is to be understood therefore that the LSP control system 12is configured differently to the cruise control system 16, for which awheel slip event has the effect of overriding the cruise controlfunction so that manual operation of the vehicle 100 must be resumed, orspeed control by the cruise control system 12 resumed by pressing theresume button 173R or set-speed button 173.

In a further embodiment of the present invention (not shown) a wheelslip signal 48 is derived not just from a comparison of wheel speeds,but further refined using sensor data indicative of the vehicle's speedover ground. Such a speed over ground determination may be made viaglobal positioning (GPS) data, or via a vehicle mounted radar or laserbased system arranged to determine the relative movement of the vehicle100 and the ground over which it is travelling. A camera system may beemployed for determining speed over ground in some embodiments.

At any stage of the LSP control process the user can override the LSPfunction by depressing the accelerator pedal 161 and/or brake pedal 163to adjust the vehicle speed in a positive or negative sense. However,absent any override by a user, in the event that a wheel slip event isdetected via signal 48, the LSP control system 12 remains active andcontrol of vehicle speed by the LSP control system 12 is not terminated.As shown in FIG. 4, this may be implemented by providing a wheel slipevent signal 48 to the LSP control system 12 which is then managed bythe LSP control system 12 and/or brake controller 13. In the embodimentshown in FIG. 1 the SCS 14S generates the wheel slip event signal 48 andsupplies it to the LSP control system 12 and cruise control system 16.

A wheel slip event is triggered when a loss of traction occurs at anyone of the vehicle wheels. Wheels and tyres may be more prone to losingtraction when travelling for example on snow, ice, mud or sand and/or onsteep gradients or cross-slopes. A vehicle 100 may also be more prone tolosing traction in environments where the terrain is more uneven orslippery compared with driving on a highway in normal on-roadconditions. Embodiments of the present invention therefore findparticular benefit when the vehicle 100 is being driven in an off-roadenvironment, or in conditions in which wheel slip may commonly occur.Manual operation in such conditions can be a difficult and oftenstressful experience for the driver and may result in an uncomfortableride.

The vehicle 100 is also provided with additional sensors (not shown)which are representative of a variety of different parameters associatedwith vehicle motion and status. The signals from the sensors provide, orare used to calculate, a plurality of driving condition indicators (alsoreferred to as terrain indicators) which are indicative of the nature ofthe terrain conditions over which the vehicle is travelling. Suitablesensor data may be provided by inertial systems unique to the LSP or HDCcontrol system 12, 12HD or systems that form part of another vehiclesub-system such as an occupant restraint system or any other sub-systemwhich may provide data from sensors such as gyros and/or accelerometersthat may be indicative of vehicle body movement and may provide a usefulinput to the LSP and/or HDC control systems 12, 12HD.

The sensors on the vehicle 100 include sensors which provide continuoussensor outputs to the VCU 10, including wheel speed sensors, asmentioned previously and as shown in FIG. 1, and other sensors (notshown) such as an ambient temperature sensor, an atmospheric pressuresensor, tyre pressure sensors, wheel articulation sensors, gyroscopicsensors to detect vehicular yaw, roll and pitch angle and rate, avehicle speed sensor, a longitudinal acceleration sensor, an enginetorque sensor (or engine torque estimator), a steering angle sensor, asteering wheel speed sensor, a gradient sensor (or gradient estimator),a lateral acceleration sensor which may be part of the SCS 14S, a brakepedal position sensor, a brake pressure sensor, an accelerator pedalposition sensor, longitudinal, lateral and vertical motion sensors, andwater detection sensors forming part of a vehicle wading assistancesystem (not shown). In other embodiments, only a selection of theaforementioned sensors may be used. Other sensors may be useful inaddition or instead in some embodiments.

The VCU 10 also receives a signal from the steering controller 170C. Thesteering controller 170C is in the form of an electronic power assistedsteering unit (ePAS unit). The steering controller 170C provides asignal to the VCU 10 indicative of the steering force being applied tosteerable road wheels 111, 112 of the vehicle 100. This forcecorresponds to that applied by a user to the steering wheel 171 incombination with steering force generated by the ePAS unit 170C.

The VCU 10 evaluates the various sensor inputs to determine theprobability that each of a plurality of different control modes (drivingmodes) for the vehicle subsystems is appropriate, with each control modecorresponding to a particular terrain type over which the vehicle istravelling (for example, mud and ruts, sand, grass/gravel/snow).

If the user has selected operation of the vehicle in an automaticdriving mode selection condition, the VCU 10 then selects the mostappropriate one of the control modes and is configured automatically tocontrol the subsystems according to the selected mode. This aspect ofthe invention is described in further detail in our co-pending patentapplication nos. GB2492748, GB2492655 and GB2499252, the contents ofeach of which is incorporated herein by reference.

The nature of the terrain over which the vehicle is travelling (asdetermined by reference to the selected control mode) may also beutilised in the LSP control system 12 to determine an appropriateincrease or decrease in vehicle speed. For example, if the user selectsa value of LSP_set-speed that is not suitable for the nature of theterrain over which the vehicle is travelling, the system 12 is operableto automatically adjust the vehicle speed downwards by reducing thespeed of the vehicle wheels. In some cases, for example, the userselected speed may not be achievable or appropriate over certain terraintypes, particularly in the case of uneven or rough surfaces. If thesystem 12 selects a set-speed that differs from the user-selectedset-speed, a visual indication of the speed constraint is provided tothe user via the LSP HMI 20 to indicate that an alternative speed hasbeen adopted.

In the present embodiment, the LSP control system 12 is configured tomonitor signals indicative of an amount of tractive force availablebetween each wheel 111, 112, 114, 115 and a driving surface. The LSPcontrol system 12 accomplishes this by receiving the following signalsas shown in FIG. 7:

-   -   (a) Vehicle reference speed signal 36;    -   (b) Wheel speed signals 36W;    -   (c) Signal sfc_mu indicative of surface coefficient of friction        between each wheel and a driving surface;    -   (d) Signal amb_temp indicative of an ambient air temperature        external to the vehicle;    -   (e) Signal sfc_grad indicative of a gradient of a driving        surface over which the vehicle 100 is driving;    -   (f) Signal wade_depth indicative of a depth of liquid in which a        vehicle is wading when a vehicle is determined to be wading, a        value of substantially zero corresponding to a non-wading        situation; and    -   (g) Signal susp_art indicative of an amount of articulation of a        suspension system of the vehicle in respect of each wheel.

The LSP control system 12 also receives a signal hi-lo indicativewhether the PTU 131P is in a HI or LO ratio and a signal tq_interyindicative that a torque intervention event is in progress in which theABS function, TCS 14T or SCS 14S intervenes to change an amount oftorque applied to one or more wheels.

The LSP control system 12 is configured to estimate an amount oftractive force that may be developed between each road wheel 111, 112,114, 115 and the driving surface and generate a transmission controlsignal LSP_trans in dependence on the estimate.

The signal LSP_trans is output to a transmission controller 124C as alsoshown in FIG. 7. The transmission controller 124C is configured toreceive the signal LSP_trans. In addition the transmission controller124C receives a signal acc_pos indicative of an accelerator pedalposition and a signal eng_spd indicative of an instant speed of rotationof the output shaft of the engine 121.

It is to be understood that, when the LSP control system 12 is in theactive mode the LSP control system 12 generates a virtual acceleratorpedal position signal acc_pos_virt which is fed to the transmissioncontroller 124C and powertrain controller 11 in addition to the signalacc_pos. The powertrain controller 11 develops an amount of powertraintorque in dependence on the accelerator pedal position signal acc_pos orvirtual accelerator pedal position signal acc_pos_virt, whichever is thegreatest, when the LSP control system 12 is in the active mode.

The transmission controller 124C is configured to command thetransmission 124 to assume a required gear according to shift map datastored in a memory of the transmission controller 124C in dependence onacc_pos or acc_pos_virt. The shift map data is stored in the form of alook-up table LUT that allows the controller 124C to determine the gearthat the transmission 124 should be operated in at a given moment intime. The data is stored in the form of values of engine output shaftspeed and accelerator pedal position (acc_pos or acc_pos_virt) as aproportion of maximum allowable range of travel of the accelerator pedal161, at which a transmission gear upshift or downshift should takeplace. The data stored is plotted graphically in FIG. 8, where downshiftpoints are shown with dashed lines and upshift points are shown withsolid lines.

By way of example of operation of the transmission controller 124C, ifthe vehicle 100 is travelling under steady state conditions with anengine output shaft speed of 3000 rpm and a virtual accelerator pedalposition of 80% of maximum stroke or range (point P1 of FIG. 8), thetransmission 124 may be in (say) gear number 4, each transmission gearcausing a predetermined gear ratio to be established between input andoutput shafts of the transmission 124 in the conventional manner. If theengine output shaft speed falls to around 2200 rpm (point S_D), thetransmission controller 124C forces a downshift in gear. Thetransmission 124 therefore assumes gear number 3, causing a larger gearratio to be established between the input and output shafts of thetransmission 124.

The transmission controller 124C receives the signal LSP_trans from theLSP control system 12. The signal LSP_trans may be in the form of adownshift_modify signal, downshift_suspend signal or downshift_tempsignal.

The downshift_modify signal causes the value of engine output shaftspeed at which a given downshift occurs, for a given value ofacc_pos_virt and eng_spd, to be reduced by the transmission controller124C by a predetermined amount that is indicated by the signaldownshift_modify.

The downshift_suspend signal causes the transmission controller 124C tosuspend all downshifts by the transmission 124 until the signaldownshift_suspend is terminated. Other arrangements for suspendingdownshifts to lower output shaft speed values may also be useful. Otherarrangements for suspending downshifts altogether may also be useful.

In response to receipt of signal downshift_temp, the transmissioncontroller 124C performs a mu-check operation in which the transmissioncontroller 124C forces the transmission 124 to downshift by one gearnumber from the current operating gear for a predetermined perioddownshift_temp_period before reverting to the current operating gear.Thus, in response to signal downshift_temp the transmission controller124C forces the transmission 124 to downshift by one gear number for 0.5s before upshifting by one gear number. In the present embodiment theperiod downshift_temp_period is of length 0.5 s although other lengthsare also useful such as 0.25 s, 1 s or any other suitable length. Insome embodiments the transmission controller 124C may cause a downshiftby more than one gear number when s mu-check operation is performed,optionally in dependence on one or more conditions such as the value ofa current gear number, the value of sfc_mu_LSP described below, or anyother suitable condition.

Whilst the transmission 124 is in the downshifted state during amu-check operation, the LSP control system 12 monitors a change in speedof any of the vehicle road wheels by reference to the wheel speedsignals 36W. If the LSP control system 12 determines that a speed of anyof the wheels is no longer consistent with vehicle reference speedsignal 36 due to an excessive reduction in wheel speed, the LSP controlsystem 12 updates an estimate of the surface coefficient of frictionbetween each wheel and a driving surface, sfc_mu_LSP. The parametersfc_mu_LSP is maintained by the LSP control system 12 independently ofthe value of parameter sfc_mu that is received by the LSP control system12 from the VCU 10. The LSP control system 12 may employ the valuesfc_mu_LSP in preference to the value sfc_mu_received by the LSP controlsystem 12 from another source such as a brake controller 13 or othercontroller when deciding whether to adjust a speed at which atransmission downshift may take place.

In the present embodiment the LSP control system 12 is configured totransmit the signal downshift_temp to the transmission controller 124Cat the expiry of a predetermined interval downshift_temp_interval sincethe last issuance of a downshift_temp signal, or after the vehiclecovers a predetermined distance of downshift_temp_distance since thelast issuance of a downshift_temp signal, whichever event occurs first,and the LSP control system 12 is issuing a virtual accelerator pedalposition signal acc_pos_virt having a value that corresponds to pedallift-off, i.e. to a released position of the pedal 161. In the presentembodiment downshift_temp_interval is of length 300 s (5 minutes) anddownshift_temp_distance is 1000 m. Other values ofdownshift_temp_interval and downshift_temp_distance may also be useful,such as 60 s and 1 mile (approximately 1.6 km). When the LSP controlsystem 12 determines that a mu-check operation is due, the system 12causes the mu-check operation to be performed at the earliestopportunity, i.e. when the condition is met that acc_pos_virtcorresponds to a released accelerator pedal 161.

It is to be understood that if the signal tq_interv indicates that atorque intervention event is occurring, the LSP control system 12 isconfigured to perform a mu-check operation at the earliest opportunityonce the signal tq_interv indicates that a torque intervention event isno longer occurring. Optionally the system 12 may perform a mu-checkonce the signal tq_interv indicates that a torque intervention event isno longer occurring even if the signal acc_pos_virt does not correspondto a released position of the accelerator pedal 161.

When the LSP control system 12 transmits the signal downshift_temp tothe transmission controller 124C, the system 12 monitors the vehiclespeed signal 36 and wheel speed signals 36W and determines whether anydifference between the vehicle speed signal 36 and wheel speed signals36W is indicative of locking of a wheel due to increased rotationalresistance of a wheel when the transmission 124 is in a lower gear. Insome embodiments if when a mu-check operation is in progress a speed ofone or more wheels falls to a value that is lower than vehicle referencespeed 36 by more than a predetermined proportion of the vehiclereference speed 36, for example that is less than 50% of vehiclereference speed, the LSP control system 12 reduces the estimate ofsurface coefficient of friction sfc_mu_LSP by a predetermined amount,optionally by an amount corresponding to the proportion of vehiclereference speed 36 by which wheel speed has decreased, such as 50% inthe case of a 50% reduction in wheels speed. In the case that wheel lockis experienced, i.e. a wheel becomes substantially stationary, thesystem 12 may set the value of sfc_mu_LSP to a minimum allowablebaseline value such as 0.1. Other values are also useful. It is to beunderstood that reducing the value of sfc_mu_LSP has the effect ofreducing an estimated value of tractive force between the drivingsurface and road wheel, tract_force_LSP, since the tractive forceavailable at a given wheel is a function of surface coefficient offriction and normal force acting on the wheel.

The LSP control system 12 is configured to further refine the value ofsfc_mu_LSP by reference to the signal amb_temp. If the value of amb_tempfalls below a predetermined value such as 3 C., the LSP control system12 may reduce the value of sfc_mu_LSP by a predetermined amount, forexample by a predetermined value such as 0.1, 0.2 or any other value, orby a predetermined proportion of the instant value of sfc_mu_LSP. Otherarrangements are also useful.

The LSP control system 12 is further configured to refine the value oftract_force_LSP in respect of each wheel by reference to the signalssusp_art, wade_depth and sfc_grad which are indicative of a normal forceacting on each wheel.

In the case of signal susp_art, the LSP control system 12 is configuredto increase or decrease the estimate of tract_force_LSP in dependence onwhether the signal indicates that a suspension arm in respect of a givenwheel is deflected in a positive sense (corresponding to greater weighton a given wheel) or negative sense (corresponding to less weight on agiven wheel) with respect to a reference position of the suspension.Positive deflections may result in an increase in tract_force_LSP for agiven wheel, whilst negative deflections may result in a decrease intract_force_LSP for a given wheel. This feature may be particularlyuseful when a vehicle is negotiating very rough terrain, and thesuspension becomes highly deflected, or highly ‘articulated’, increasinga risk that one or more wheels may suffer excessive slip.

The signal wade_depth is employed to determine an apparent weight of thevehicle 100 taking into account buoyancy, and thereby reduce the valueof tract_force_LSP accordingly.

The signal sfc_grad is employed to determine a change in distribution ofweight over wheels of the vehicle 100 due to the gradient, since aproportion of a weight of the vehicle borne by downhill wheels typicallyincreases relative to the proportion borne by uphill wheels. An amountof tractive force available at downhill wheels may therefore beincreased, and/or that available at uphill wheels reduced.

The signal hi_lo may be employed to determine an overall driveline gearratio and enable correlation of engine output shaft speed with vehiclespeed.

Throughout the description and claims of this specification, the words“comprise” and “contain” and variations of the words, for example“comprising” and “comprises”, means “including but not limited to”, andis not intended to (and does not) exclude other moieties, additives,components, integers or steps.

Throughout the description and claims of this specification, thesingular encompasses the plural unless the context otherwise requires.In particular, where the indefinite article is used, the specificationis to be understood as contemplating plurality as well as singularity,unless the context requires otherwise.

Features, integers, characteristics, compounds, chemical moieties orgroups described in conjunction with a particular aspect, embodiment orexample of the invention are to be understood to be applicable to anyother aspect, embodiment or example described herein unless incompatibletherewith.

1. A system comprising: a first controller configured in each of apredetermined plurality of states to generate a torque request signal inorder to cause a vehicle to operate in accordance with a target speedvalue, the system being configured, when the first controller is one ofsaid plurality of states to generate a gear shift limit signal to causea change in an engine speed at which a transmission controller ispermitted to cause a gear downshift, the gear shift limit signal beinggenerated in dependence on at least one traction indicator signalindicative of an amount of estimated traction between the vehicle and adriving surface, wherein the gear shift limit signal is furtherconfigured to suspend a gear downshift if the estimated traction betweenthe vehicle and a driving surface is below a first threshold. 2.(canceled)
 3. A system according to claim 1 wherein suspending a geardownshift comprises reducing the engine speed at which a transmissioncontroller is permitted to cause a gear downshift to substantially zero,whilst the vehicle is in motion.
 4. A system according to claim 1wherein the gear shift limit signal is configured to reduce the enginespeed at which a transmission controller causes a downshift.
 5. A systemaccording to claim 1 wherein the gear shift limit signal is configuredto substantially prevent a transmission controller from causing adownshift by reducing the engine speed at which a transmissioncontroller causes a downshift.
 6. A system according to claim 1 whereinthe at least one traction indicator signal is a signal indicative of oneor more of: a normal force between a wheel and a driving surface; anamount of torque that may be applied to a wheel before slip of the wheeloccurs; and a surface coefficient of friction between a wheel and adriving surface. 7-8. (cancelled)
 9. A system according to claim 1wherein the first controller is configured to perform a mu-checkoperation when in one of the plurality of states, the system beingconfigured to perform a mu-check operation by causing a transmissioncontroller to cause a downshift and monitoring a speed of one or morewheels in response to the downshift.
 10. A system according to claim 9wherein when performing a mu-check operation the first controller isconfigured to calculate an estimated value of surface coefficient offriction between a wheel and a driving surface in dependence on a changein a speed of one or more wheels in response to the downshift.
 11. Asystem according to claim 9 configured to perform a mu-check operationwhen one or more predetermined conditions are met during operation inone of said plurality of states.
 12. A system according to claim 11configured to perform a mu-check at predetermined intervals of time. 13.A system according to claim 6 configured to perform a mu-check followinga torque control intervention event in which a torque interventionsystem intervenes to cause change in an amount of torque applied to oneor more wheels.
 14. A system according to claim 1 wherein the at leastone traction indicator signal is indicative of an amount of torque thatmay be applied to a wheel before slip of the wheel slip exceeding apredetermined amount occurs.
 15. A system according to claim 1 furtherconfigured, when causing a vehicle to operate in accordance with thetarget speed value, to generate a brake request signal to command abrake system to deliver an amount of brake torque.
 16. A systemaccording to claim 15 wherein said plurality of states includes a firstone or more first states in which the system is configured to generate apowertrain request signal in order to cause a powertrain to deliverdrive torque and cause a vehicle to operate in accordance with a targetspeed value, and a second state wherein the system is configured tocause a vehicle to operate in accordance with a target speed value bygenerating the brake request signal and not to generate a powertrainrequest signal in order to cause a powertrain to deliver drive torque.17. A motor vehicle comprising a system according to claim
 1. 18. Avehicle according to claim 17 further comprising a chassis, a bodyattached to said chassis, a plurality of wheels, a powertrain to drivesaid wheels, and a braking system to brake said wheels.
 19. A method ofoperating a vehicle implemented by means of a control system, the methodcomprising: causing a powertrain to deliver drive torque and cause avehicle to operate in accordance with a target speed value; generating agear shift signal to cause a change in a speed at which a transmissionis permitted to downshift or substantially prevent a transmission fromdownshifting, and generating the gear shift signal in dependence on atleast one traction indicator signal being a signal indicative of anamount of tractive force that may be developed between a driving surfaceand one or more driving wheels, wherein the gear shift limit signal isfurther configured to suspend a gear downshift if the estimated tractionbetween the vehicle and a driving surface is below a first threshold.20. A non-transient carrier medium carrying computer readable code forcontrolling a vehicle to carry out the method of claim
 19. 21. Acomputer program product executable on a processor so as to implementthe method of claim
 19. 22. A computer readable medium loaded with thecomputer program product of claim
 20. 23. A processor having anassociated memory with instructions stored therein, the processorarranged to access the memory and execute instructions stored therein soas to implement the method of claim
 18. 24. (canceled)